1. Field of the Invention
The present invention relates to cables of the type employed for communication of optical and/or electrical signals between a ship and sensors of the hydrophone type. More particularly this invention pertains to an improved wet end termination for such a cable.
2. Description of the Prior Art
SONAR-based systems for detecting underwater hazards and threats employ pressure-actuated transducers of the hydrophone type to generate signals indicative of the presence and location of underwater objects. Such objects are "illuminated" by the reflection of acoustic wavefronts that are initially transmitted from a ship. The acoustic signals, when reflected, provide phase shift information that, when processed, enables one to ascertain underwater position.
In use, a SONAR system utilizes a predetermined submergeable arrangement of acoustic hydrophones to collect a spatial distribution of "echo" data that can be analyzed for such information as position and closing rate.
The array is housed within a hose-like element that is fixed to the end of a towing cable comprising an arrangement of optical fibers and/or electrical conductors within a protective outer jacket. An example of such a cable is disclosed in U.S. Pat. No. 4,952,012 of Stamnitz entitled "Electro-Opto-Mechanical Cable For Fiber Optic Transmission Systems."
The processing of data from an array is well-known and discussed, for example by A. Dandridge et al. in the article "Multiplexing of Interferometric Sensors Using Phase Carrier Techniques," Journal of Lightwave Technology, Vol. LT-5, No. 7 (July 1987) at pages 947-952.
While the towed array is a well accepted and recognized element of a SONAR system, its realization in practice is fraught with a number of serious mechanical difficulties. Some of such problems are related to deployment of the array while others pertain to the maintenance of reliable optical and/or electrical contact between it and the SONAR equipped ship.
The cable for a towed array may have a outer diameter of approximately one half inch, enclosing a plurality of optical fibers and/or a coaxial electrical optical conductor. The cable arrangement described in the Stamnitz patent comprises a 0.6 inch steel "armoring" cable that includes two layers of armoring wrapped around a central member containing the fiber optics. Eighteen (18) 250 micron buffered fibers are enclosed within a tube or jacket. The eighteen optical fibers provide optical communication between the hydrophones of the array and the on-board electronics. An example of an alternative cable arrangement for a towed array might include a 0.6 inch steel armored cable including two layers of armoring wrapped around a central member containing the fiber optics and a coaxial wire. In one such arrangement, seven steel tubes contain eighteen (18) 250 micron buffered fibers with one large tube enclosing twelve fibers, six small tubes enclosing one fiber each and a coaxial line. The single-fiber tubes are helixed around the large tube. Other arrangements may, of course, be utilized for communication between the towed array and the ship electronics.
The armoring and jacketing of the optical fibers is employed to protect the relatively-fragile fibers and/or electrical conductor. Such conductors and fibers require protection in view of the substantial stresses that can be incurred during both deployment and use of the array. A typical array may weigh in the neighborhood of 200 pounds and require a cable weighing 2,000 pounds or more. A ship towing the array at twenty-five (25) knots can cause such an arrangement to be subjected to significant stresses that will be felt most acutely at the distal underwater or "wet" end of the cable in the region of joinder to the towed array.
The stressing of the cable and array demands special care at the wet-end termination where the cable's conductors are joined to those of the towed array. As mentioned earlier, the hydrophones of the towed array are enclosed within a flexible hose. A fill fluid such as ISOPAR L, a trademark of the Exxon Corporation, selected for its acoustic properties to assure that the reflected waves are transmitted to the hydrophones within the hose-like structure, fills the interior of the hose. Typically, the conductors of the multiplexed hydrophone network are sturdier than those of the cable, such conductors being formed, for example, of 900 micron thick HYTREL (a trademark of the DuPont Corporation) coated fiber.
In the prior art, the cable is typically terminated with a coupling module that encloses optical fusion splices between the cable conductor and the 900 micron HYTREL coated fiber of the towed array. The splices are encapsulated with an appropriate compound for protection from the fill fluid, which is quite caustic. Additional optical fiber is provided in the form of a loop to take up some stressing of the cable's outer jacket. The interior of the coupling module is filled with open cell foam, providing a snug fit.
Prior art arrangements such as described above do not always provide sufficient protection from the often-hostile environment. By constraining the fiber inside the packed foam, the freedom of the fiber of the loop to extend axially within the cable and thereby absorb some stressing is hindered. This effectively defeats the margin of safety provided by the loop. Furthermore, the coupling module, typically formed of rubber or other hose-like material, does not provide adequate structural support when the termination is reeled or placed under other stresses. On board, the cable is stored on a winch, requiring flexibility. The cable must hold up to rough handling during and after deployment. The hose-like coupling modules of the prior art leave the wet end subject to damage when grasped near the interconnection of the conductors of the cable to those of the array since jacketing must be stripped from the cable to allow splicing to the towed array. As a result, this portion of the cable is relatively vulnerable to damage due to squeezing and the like.